Understanding your vessel’s natural roll period is a useful point of measure in understanding the science of stabilization.
More owners are understanding and appreciating the benefits of “active” stabilizers to ensure comfort and safety on board. Understanding your vessel’s natural roll period is a useful point of measure in understanding the science of stabilization. A ship experiences its largest roll angles at or near its natural roll period.
The natural roll period is simply the time it takes for the ship in calm seas and no wind to complete roll to port, then starboard, and return upright. It is directly related to the metacentric height (GM) and beam of a vessel. The metacentric height is dependent upon:
- The hull shape
- The displacement of the vessel, which changes with the loading condition
- The vessel’s center of gravity, which changes with the loading condition and activity aboard the vessel
The metacentric height is the distance between the center of gravity and the metacenter. An approximation of the vessel’s roll period can be made with the following simple formulas:
Comfort and Seasickness are Impacted by Acceleration and the Roll Period
The acceleration involved in a vessel roll is typically the cause of seasickness. Most people are comfortable with a roll period of eight to 10 seconds. Conversely, most people are uncomfortable and suffer from peak seasickness sensitivity with roll periods around five seconds. Short roll periods feel snappy and are usually the root cause of discomfort because the vessel responds quickly to a roll motion, i.e. acceleration. Long roll periods, though not the contributor to seasickness, are usually scary because the vessel feels more unstable.
Roll Period = Wave Period → Bad Combination!
Be aware of the wave period and stay clear of the danger zone where the natural roll period is the same as the wave period. The wave period is the time it takes for one wave cycle (crest and trough) to pass through a stationary point. Wave period reports can be collected from most maritime weather forecasts, such as this sample for the Fort Lauderdale area:
West northwest winds 20 to 25 knots. Seas five to seven feet with occasional seas to nine feet. Dominant period five seconds.Intracoastal waters a moderate chop.
Your best options:
- Avoid areas where your roll period = the wave period
- Find a location with longer or shorter wave periods
- Find a location with lower wave slopes
- Increase the vessel speed
- Change the heading into the seas
- Increase damping with stabilizers or bilge keels.
This column originally ran in the January 2021 issue of Dockwalk.